Antibody diversity may arise by random combination of VH-D-J and VL genes and by somatic mutation. In this regard amino acid sequences and serologic identification of idiotypes (Id) of structurally and functionally related antibodies have been extensively studied. Phosphoryl choline (PC) binding myeloma proteins (MP) and hybridoma proteins (HP) have been especially useful in studies of Id heterogeneity. In the mouse at the DNA level studies of PC binding MP or HP show that the entire VH repertoire is generated from a T15 germ line gene and that somatic mutations account for the VH variations in M167 or M603 MP. Based on sequence studies, VL regions of PC binding MP fall into three kappa sub-groups: T15 (VK22), M167 (VK24) and M603 (VK8). PC antibodies are constructed by association of any of these light chains with the VH4 (T15 heavy chain germ line sequences). Serologically defined idiotopes have been identified for each group. One of the more complex problems is the relationship of the primary amino acid sequence to the confor6ational structure in the expression of an idiotope. In the studies presented we prepared monoclonal anti C3Id antibodies (NL-16 and NL-24). C3Id is associated with a CBBPC3 MP from a C.BB22 mouse and is identical in amino acid sequence to T15 MP except for four VH framework amino acids and a different constant region. NL16 antibody identified an Id (C3-16) determined by the D-region (YYGSS) and the VK22 L-chains of T15. C3-16Id was found to be associated with the PC combining site suggesting that the D-region conformational structure and light chains effected the Id function. NL-24 antibody identified an Id (C3-24) present on C3, T15 and H8 but not on light chain unrelated M603 or M167 PC binding MP. Only T15Id+ IgA and not IgG or IgM PC binding MP or HP expressed C3-24 Id. However, T15Id- HP did not express C3-24 Id. This C3-24 Id requires T15I3X (VK22-VH4) and IgA CH regions to be expressed in PC binding antibodies. Anti PC serum from a variety of inbred strains did not inhibit C3-24. However, when IgA fraction was isolated from a pool of anti PC serum of BALB/c mice, 70% inhibition of NL-24 and C3 binding occurred.